1. Field of the Invention
The present invention relates to mechanical-joint connections for use with gray iron or ductile iron pipe for water and other liquids.
2. Description of the Prior Art
Gray iron and ductile iron pipes in nominal sizes ranging from 2 inches to 48 inches are extensively used in systems handling water and other liquids at pressure ratings from 150 to 350 pounds per square inch. Conventional fittings for such pipes are of three general types: mechanical-joint, bell-and-spigot, and flanged.
A mechanical joint is a gasketed type of connection for joining sections of gray iron or ductile iron pipe. It comprises three elements -- a flanged bell, a rubber gasket, and a compression ring or gland. The bell is normally cast as an integral part of one end of a standard length of pipe or as an integral part of a connecting piece, sleeve, bend, tee, or other fitting having specified standard diemnsions for each nominal pipe size. The bell end of one pipe section is adapted to slip over a plain end of another pipe section of the same nominal size, after first slipping a flanged gland and a rubber gasket over the plain end. The gasket fits inside the bell and is maintained in compression by bolting the flange of the gland to the flange of the bell.
Two sections of pipe connected by means of such a mechanical bell joint are held together by the friction of the rubber gasket. The mechanical-joint type of pipe connection offers the advantage of limited flexibility to accommodate up to about 5.degree. of angular deflection between adjacent pipes without leakage.
As mentioned above, the bell of a mechanical-joint connection is normally cast as an integral part of a full length of pipe (18 feet or 20 feet standard laying length) and normally on one end only, with the other end being left plain. In addition, there are a number of standard dimension fittings, such as bends, tees, reducers, sleeves, connecting pieces, and so forth that incorporate an integrally cast mechanical-joint bell on at least one end. The standard sleeves and connecting pieces are intended to provide transitions between different types of pipe, such as between steel pipe and cast-iron pipe or between fittings of one connector type and another connector type (e.g., mechanical-joint to bell-and-spigot or mechanical-joint to flanged). In all cases where a mechanical-joint bell end is provided on a standard pipe or fitting, however, the bell end is cast integrally with the pipe or fitting.
Most piping layouts require pipe lengths shorter than the standard laying lengths in various parts of the system, however. Particularly in piping systems involving the larger sizes of pipe, to provide a mechanical-joint bell end on lengths of pipe shorter than the standard laying lengths becomes expensive, because the cut-off portion must normally be scrapped. For example, the present cost of 36-inch cast iron pipe is approximately $55 per lineal foot. Thus, if only a short length of bell-ended pipe is needed, the value of the remainder of a 20-foot standard pipe that has been cut off may run into many hundreds of dollars.
An important application for cast iron pipe in the larger sizes is in sewage treatment and water treatment plants. Sewage treatment plants have many large concrete sludge tanks, and these tanks have a number of pipe penetrations through their walls. Pipe penetrations are also required through the walls of plant buildings. The walls of these tanks and buildings may be of varying thickness, and each pipe penetration requires a wall sleeve casting that must be properly positioned inside the forms before the concrete for the wall is poured. A typical sewage treatment plant may require over a hundred of such wall sleeves. These wall sleeves typically are special castings having a mechanical-joint bell at one or both ends and are ordered to specified lengths, depending on the wall thickness. These special castings need extra treatment at the foundry, requiring that they be ordered far in advance. Lead times may run eight months or more. Furthermore, because these wall castings are to be embedded in concrete walls, any delay in their delivery will cause delays in the schedule for pouring concrete. This can create a ripple effect resulting in costly delays affecting entire projects, because the timely pouring of concrete is basic to maintaining the orderly progress of a job.
In addition to high cost and slow delivery, conventional wall sleeve castings present a problem when setting up the forms for tank or building walls. Typically the first stage of concrete work for a tank or building is to pour footings and a floor slab. A vertical gridwork of reinforcing steel is next set up along each side of the floor slab, and then the steel is enclosed by spaced-apart forms for containing the poured concrete of the walls.
If the concrete structure is a sewage sludge separating tank, for example, a number of wall sleeves may have to be incorporated into the form at locations as much as 15 feet or so above the floor slab. These wall sleeve castings may range up to 48 inches in diameter and weight many hundreds of pounds. Since they must be supported by the wall forms, these sleeves are installed after the reinforcing steel has been placed, with the predictable result that often the reinforcing bars have preempted the space intended for the wall castings.
In addition, installing the castings and bolting them to the forms is a time-consuming task because the reinforcing steel limits access, and bolting up the second form is necessarily a blind operation. It is also a task that involves several different trades, with the usual result that many manhours are wasted in each installation.
The conventional mechanical-joint connection has another disadvantage in that it is held together only by friction between the rubber gasket and the inner pipe as a result of the pressure exerted on the gasket by the gland ring. Although special retaining glands are available which have lock screws for positive engagement with the outer surface of the plain-ended pipe, such retaining glands are not very effective. Moreover, they cost about fifty percent more than a conventional gland.
Consequently, the usual procedure is to provide heavy thrust support foundations at locations, such as elbows and tees, where the pipe joints are subjected to net axial forces tending to separate the joint. The need for such large and heavy foundation structures adds significantly to the cost of water and sewage treatment plants.